In the commercial production of lubricating oils, particularly hydrocarbon oils of lubricating viscosity, for example, viscosities in the range about 100 to 1,000 SUS at 100.degree. F., such oils containing dissolved additives of various kinds and the additives themselves, a terminal step in the process of manufacture is frequently filtration and in the filtration a filter aid which is siliceous in character being a material such as diatomaceous earth, fuller's earth, etc. is employed. During the filtration filter cakes are produced which consist of a filter aid and a very appreciable amount of contained oil or additive solution in oil. Filter aids of this kind have high surface areas and adsorb organic compounds, usually in amounts about equal to their own weight. Disposal of such filter cakes as an industrial waste presents two problems. First, there is an appreciable loss of valuable product contained in the filter cakes and increasing environmental concerns have made it impossible to simply dump the filter cake and have required that it be treated in some manner to reduce the oil content before dumping or that it be disposed of at an approved waste disposal site.
Such filter cakes are frequently washed with an organic solvent to remove the heavier oils contained on the cake, but this results in the displacement of one organic material with another and is not a very satisfactory solution to the problem. The filter cakes also have been subjected to incineration during which the contained oil is burned and the residual clay is then disposed of. The products of combustion, however, require processing before they are released to the atmosphere and this increases cost of disposal and, of course, involves the loss of the valuable contained material.
Proposals have been made to treat such filter cakes to recover contained oil. Lorenz et al U.S. Pat. No. 3,835,021 describes filtration of refinery oil sludges, using diatomaceous earth as a filter aid to separate the oil from the sludge. The filter cake produced durin9 such filtration is then washed with water at 125.degree. F. to 200.degree. F. to remove a substantial part of the oil and the filter cake is then discarded.
Chorney U.S. Pat. No. 3,725,467 describes treatment of a filter cake produced by filtering an oil solution of a calcium mahogany sulfonate additive, using diatomaceous earth as a filter aid, with an aqueous solution of certain acids whose calcium salts are water soluble to recover a substantial part of the organic materials contained in the filter cake.